In situ calibration of inductively coupled plasma-atomic emission and mass spectroscopy

ABSTRACT

A method and apparatus for in situ addition calibration of an inductively coupled plasma atomic emission spectrometer or mass spectrometer using a precision gas metering valve to introduce a volatile calibration gas of an element of interest directly into an aerosol particle stream. The present situ calibration technique is suitable for various remote, on-site sampling systems such as laser ablation or nebulization.

STATEMENT OF GOVERNMENT RIGHT

This invention was made with support of the U.S. Government under UnitedStates Department of Energy Contract No. W-7405-ENG-82. The Governmenthas certain rights in the invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relate to a method and apparatus for the in situaddition calibration of an inductively coupled plasma atomic emissionspectrometer or mass spectrometer using a precision gas metering valveto introduce a volatile calibration gas of an element of interestdirectly into an aerosol particle stream, and in particular, use of thepresent in situ calibration technique with various remote, on-sitesampling systems.

2. Description of the Related Art

Inductively coupled plasma atomic emission spectrometers and massspectrometers can be calibrated by adding a series of standardizedadditions to the sample being tested. A standard addition curve for theanalyte being measured can then be generated. However, in somecircumstances it is difficult or not possible to add the standards tothe sample.

For example, there is an ongoing need to sample and analyze dangerous orhazardous materials, or materials located in hazardous environments suchas soil or water at hazardous waste sites (radioactive wastes, toxicchemical dumps or contaminated structures) or molten metals in amanufacturing foundry. Conventionally, a sample of a hazardous waste isremoved from the site and brought to a laboratory for analysis. Thesample must therefore be carefully extracted, transported, handled andstored in order to assure the safety of the technicians carrying out thetest, as well as the public. The expense and delay entailed inextracting, handling and storing such materials, as well as the healthrisks, have encouraged scientists to develop alternative testingapproaches minimizing these disadvantages.

The Iowa State University Research Foundation has developed a system foranalyzing the composition of specimens directly at a sample site. Thissystem is disclosed in a U.S. patent application Ser. No. 08/117,242,entitled MOBILE INDUCTIVELY COUPLED PLASMA SYSTEM, filed by A. D'Silvaand E. Jaselskis, and further disclosed in published PCT application No.WO 93/07453, both of which are hereby incorporated by reference.However, the amount of a sample delivered to the inductively coupledplasma (ICP) or mass spectrometer varies with laser output power andpower density at the sample surface, light scattering from aerosolparticles in the ablation cell, and variations in aerosol transport outof the cell and through the transfer tubing to the ICP torch.

In order to quantitate and normalize on-site samples, a method andapparatus for determining in situ the mass and concentration of elementsof interest is needed for use with inductively coupled plasma atomicemission spectrometers (LA-ICP-AES) and a mass spectrometers. Variousmethods for normalization and quantitation in laser ablation samplingare described in D. Baldwin, D. Zamzow, and A. D'Silva, "Aerosol MassMeasurement and Solution Standard Additions for Quantitation in LaserAblation-Inductively Coupled Plasma Atomic Emission Spectrometry", 55Anal. Chem. 1911, 1917 (1994), which is hereby incorporated byreference.

The method disclosed in the above noted article combines the techniqueof aerosol mass measurement and solution standard additions. A portionof the laser-ablated sample aerosol is diverted to a quartz microbalanceand the mass flow rate is measured. During the laser ablation samplingprocess, a measured amount of a desolvated aerosol obtained fromultrasonic nebulization of solution standards is added to thelaser-ablation aerosol to generate a standard addition curve for theanalyte being determined.

However, ultrasonic nebulizers are not 100% efficient. Failure tonebulize all of the liquid in the standard will introduce error into thesystem. Nebulization may also be effected by temperature and pressure.Additionally, the liquid standards utilized with this techniquetypically contain dangerous concentrations of acids and can be unstableover time. Finally, the overall complexity of the pumps and valvesnecessary to introduce the nebulized standard into the ablation streamare not well suited to automation and may impact on the reliability ofthe system.

SUMMARY OF THE INVENTION

The present invention relates to a method and apparatus for the in situaddition calibration of an inductively coupled plasma atomic emissionspectrometer or inductively coupled plasma mass spectrometer using aprecision gas metering valve to introduce a volatile calibration gas ofan element of interest directly into an aerosol particle stream.

In the preferred embodiment, the aerosol mass is measured alone. Aseries of calibration gas standards of known quantity are then added tothe aerosol particle stream to generate a standard addition curve forthe analyte, so that the mass and concentration of the analyte in thesample can be determined.

The preferred method involves diverting a portion of the particle streamto amass flow rate measuring system. The mass concentration of theaerosol is measured so that the analytical signal can be normalized forthe amount of the sample introduced into the ICP. A precision gasmetering valve is used to introduce a series of accurately measuredspecimens of different quantities of a volatile calibration gas of anelement of interest into the aerosol particle mass stream. The combinedcalibration gas/particle mass stream then passes to an ion-coupledplasma torch where the particles are vaporized, atomized, and ionized toemit their characteristic emissions of optical radiation and ions of theelemental constituents. The intensity of the emissions from the testspecimens are plotted on one axis and the quantity of calibration gasadded is plotted on the other axis. The resulting plot can beextrapolated to determine the mass of the element of interest.

For the mass spectroscopy embodiment, the ions are counted by theinstrument directly. When accurately measured quantities of the elementsof interest are added to the particle stream in varying quantities, aplot of ion intensity versus quantity added can be dram. Extrapolatingthe plot to the x-axis reveals the original concentration of the elementof interest.

The combination of the volatile calibration gas and the laser-ablationaerosol generates a standard addition curve for the analyte beingmeasured. The standard addition procedure corrects for potential plasmarelated matrix effects in the ICP emission signal, allowing only onestandard test sample for calibration without the need for an internalstandard in the samples.

The volatile calibration gas maybe introduced to the particle streamanywhere between the sample location and the ICP or mass spectrometer,depending upon the configuration of the system.

According to one aspect of the invention, a remotely controlled mobilecart positions a probe proximate to the sampling site. A high energywavelength laser ablates the material, forming a cloud of micron-sizedparticles. The particles are drawn from the sampling site by an aerosolsystem which employs an inert gas, such as argon. Prior to the sampleparticles and the carrier gas being injected into an ICP source, avolatile calibration gas is introduced to the particle stream. Theresulting electromagnetic radiation can be analyzed with an opticalspectrometer to determine the concentration of the element of interest.Alternatively, an ultrasonic nebulizer may be substituted for the laserablation system.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of the present in situ calibrationprocess utilizing a volatile gaseous standard;

FIG. 2 is an exemplary graphical illustration of the additioncalibration technique of the present invention;

FIG. 3 provides a schematic illustration of an exemplary mobileinductively coupled plasma system;

FIG. 4 illustrates an exemplary application of the mobile inductivelycoupled plasma system for sampling soil at a hazardous waste site; and

FIG. 5 is a schematic illustration of an exemplary mobile inductivelycoupled plasma system utilizing an ultrasonic nebulizer rather thanlaser ablation to extract a sample.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 is a schematic illustration of the preferred in situ calibrationsystem for an inductively coupled plasma atomic emission spectroscope32, 40 (ICP-AES) or an inductively coupled plasma mass spectroscope 32,114 using a precision gas metering valve. A carrier gas 23 forces acarrier gas/particle stream 100 from an ablation cell 25 to a quartzmicrobalance 102. In the preferred embodiment, 5% to 15% of the carriergas/particle stream 100 is analyzed by the quarts microbalance 102,while the remainder of the combined stream bypasses quarts themicrobalance 102. The mass flow rate is determined based on theproportion of the carrier gas/particle stream flow diverted. Thepreferred quarts microbalance 102 consists of a piezoelectricmicrobalance mass sensor with an electrostatic precipitator to depositaerosol particles onto the sensor.

In an alternative embodiment, the entire carrier gas/particle stream 100is analyzed by the quarts microbalance 102 for a period of time. Thequarts microbalance 102 is then bypassed for the calibration phase ofthe analysis. The mass flow rate of the carrier gas/particle stream 100is assumed to remain constant for the calibration phase of the analysis.

A volatile calibration gas 104 is metered through a precision meteringvalve 106 and mixed with the carrier gas/particle stream 100 in a mixingvalve 108 by the calibration system 112. The metering valve 106 allowsthe mass flow rate of the calibration gas 104 to be accuratelycontrolled and measured. The calibration gas preferably is a volatilecompound of the element of interest. For example, silicon is a goodbaseline for soil samples between sites. If silicon is the element ofinterest, the calibration gas 104 can be silicon tetrachloride.

The combined carrier gas/particle stream/calibration gas 110 is directedto an inductively coupled plasma system (ICP) 32. The ICP 32 vaporizes,atomizes, and ionizes the particle stream and calibration gas to emitcharacteristic emissions of optical radiation or ions of the elementalconstituents. The element of interest can be isolated and quantified.The intensity of the emission from the element of interest isproportional to the quantity of the element present in the sample. If anatomic emission spectrometer 40 is used, the electromagnetic radiationfrom the ICP 32 is analyzed. Alternatively, a mass spectrometer can beused to analyze ions from the ICP 32.

A stepper motor 130 maybe added to the metering valve 106 to allow bothmanual and computer control of the valve 106. The preferred meteringvalve is available from Vacuum Accessories Corp. of America, under themodel number SMC-102A. However, it will be understood that a variety ofmetering valves are available which are suitable for this purpose.

FIG. 2 is an exemplary graphical illustration of the additioncalibration technique of the present invention in which the mass ofcalibration gas 104 added is plotted on the x-axis and the signalintensity of an atomic emission spectroscope or mass spectroscope isplotted on the y-axis. The signal intensity for the carrier gas/particlestream 100 without the calibration gas 104 is shown at point 120.Varying amounts of the calibration gas 104 are added to the carriergas/particle stream 100. The output signal intensity from the atomicemission spectrometer 40 or mass spectrometer 114 vary proportionally tocombined quantity of the element of interest present in the aerosolstream 100 and the calibration gas 104. The signal intensity for eachquantity of calibration gas 122, 124, 126 are also plotted. The x-axisintercept 128 represents the amount of mass of the element of interestin the original sample 25. The mass flow rate measured by the quartsmicrobalance 102 is used to determine the actual concentration of theelement of interest.

If a mass spectrometer 114 is substituted for the atomic emissionspectroscope 40, the ion intensity is plotted on the y-axis and the massof calibration gas added to the sample is plotted on the x-axis.Multiple accurately measured quantities of the elements of interest areadded to the particle stream in varying quantities so that a plot of ionintensity versus quantity added can be drawn. Extrapolating the plot tothe x-axis reveals the original concentration of the element ofinterest.

For example, if the element of interest is silicon, silicontetrachloride can be the calibration gas. The metering valve from VacuumAccessories Corp. of America can meter as low as 1×10⁻¹⁰ std. cc/sec.Silicon tetrachloride has a density of 7×10⁻³ g/cc which corresponds to1.7×10⁻¹³ g/sec silicon. If the sample from a 3 minute ablation totaled1 nanogram, the rate of ablation would be on the order of 5.6×10⁻¹²g/sec, which is approximately thirty (30) times the minimum meteringrate of the valve. The resulting error rate being less than 3%.

The present in situ calibration system may be used with any inductivelycoupled plasma atomic emission spectrometer or mass spectrometer inwhich a volatile calibration gas of an element of interest can bemetered directly into an aerosol particle stream. While the on-sitesampling systems disclosed herein are an important application for thepresent invention, it will be understood that the present invention isnot limited to such use.

FIG. 3 provides a simplified schematic illustration of an exemplarymobile inductively coupled plasma system 10 for on site sampling. Laserradiation from a laser 12 is directed to the sampling site 14 throughfused silica fiber optics 16. The exemplary laser provides continuous orpulsed, fixed wavelength laser radiation at least three differentwavelengths, 1064 nm, 532 nm, and 355 nm. These wavelengths are chosento provide a range of energies as materials to be analyzed havedifferent absorption characteristics at different wavelengths. Sincecurrent optical fibers are subject to damage at wavelengths below 350 nmand power levels of 10⁸ watts/cm² /sec., it is best to utilize laserwavelengths above 350 nm when using a fiber optic delivery system. Theseconstraints will change with the availability of better optical fibers.As most materials absorb optical radiation in the ultraviolet, ablationis more efficiently carried out at wavelengths below 400 nm. TheLumonics Dye Laser (Hyper-Dye 300) pumped by the Lambda Physik ExcimerLaser (model EMG102MSC) is known to provide laser beams suitable,although the preferred system for field operation is the solid state YAGlaser.

A laser focusing system 18 is provided to focus the laser output ontothe optical fiber 16, without reaching overload. Polymicro Technologiesfiber optics cable model FVPS600660690 is known to be suitable forcarrying the laser radiation to a ablation cell 20, provided no morethan 10⁸ watts/cm² /sec. is applied to the head end of the fiber. Asnoted above, power levels in excess of this can damage the fiber.Focusing system 18 may include a filter to narrow the laser beam andreduce the power actually received by the optical fiber and a series oflenses to focus the laser radiation onto the end of the optical fiber.

The ablation cell 20 has optics 22 for focusing the laser radiation fromthe fiber 16 on the material to be sampled 14. The ablation cell 20 isgenerally constructed of aluminum, but other materials maybe preferableto contend with different environmental conditions. A carrier gas source23 preferably supplies argon gas 24 to a ablation cell sampling chamber26 through an aerosol input line 28, however other gases may be suitablefor this purpose with the appropriate ICP. The material ablated orsampled 25 by the laser radiation mixes with the argon 24 to form anaerosol which is drawn from the ablation cell sampling chamber 26through the aerosol output line 30 to the inductively coupled plasma(ICP) source 32. Argon 24 is the support gas for the ICP 32. The presentinvention employs an RF Plasma Products® inductively coupled argonplasma system.

The aerosol is directed into the plasma source 34, through an outputline 30 to the ICP 32. The energized sample particles are vaporized,atomized, and ionized to provide characteristic optical reduction of theelemental constituents of the sample 25 in the form of electromagneticradiation 37, which is focused by a lens 36 and thereby subsequentlychanneled through an ICP output optical cable 38 to a multi-channel orsequential optical spectrometer 40. Alternatively, the laser light maybe directly delivered to the spectrometer 40. The present calibrationsystem 112 is located upstream of the ICP 32.

To carry the optical output of the ICP 32 to the spectrometer 40, thepreferred embodiment of the present invention employs PolymicroTechnologies fiber optic bundle (model PTA-EI0019FF-030-0DP), consistingof 19 separate 200 μm core diameter fibers arranged in a round-to-linearbundle. The Acton Research Corp. 0.5 meter spectrometer (model VM-505)equipped with a 2400 grooves/mm grating has been found suitable as thespectrometer. The optical radiation dispersed in the spectrometer isdetected by a multichannel diode array detector 41. The EG&G PrincetonApplied Research intensified diode array (model 1420) and diode arraycontroller (model 1463) are known to be suitable for this purpose.Preferably, the IEEE output of the detector 41 is connected to apersonal computer 42 or work station whereby the output of thespectrometer 40 can be stored, enhanced, processed, analyzed, anddisplayed.

FIG. 4 illustrates an exemplary embodiment of the mobile inductivelycoupled plasma system 10. A remotely controlled mobile cart 60 iscarried on a trailer 62 behind a truck 64. The truck 64 contains a powersource for operating the components of the system. In use, the truck 64is positioned a distance from the toxic waste sampling site 14. Theremotely controlled mobile cart 60 is then positioned proximate to thesampling site 14, for instance a sampling bore 68 adjacent to the toxicwaste storage chamber 70, by direct visual reckoning or by use of videoimages relayed from a video camera (not shown) mounted on the cart 60.The controls for maneuvering cart 60 are located in the truck 64.

The remotely controlled mobile cart 60 carries the ICP source 32, sothat the ICP source is as close to the sampling site 14 as possible,thereby minimizing the distance the hazardous material needs to betransported in the aerosol output line 30 and to keep the hazardousmaterial away from the operators positioned in the truck 64.

The aerosol tubes 28 and 30 are 0.25" in diameter, made of Teflon® orpolyethylene material, and are pressurized to provide a gas flow of 1.0liters/minute. The argon 24 is held under pressure in the argon source23 to provide pressure to the system. Transportation of material samples25 in the aerosol line 30 to a distance of 100 feet has been achieved.

The laser source 12, spectrometer 40, and present calibration system 112are located in the truck 64. As explained above with respect to FIG. 3,an optical fiber 16 carries the laser beam from the laser 12 to theablation cell 20, while a second fiber 38 carries the output of the ICP32 to the spectrometer 40. Using the equipment specified herein, thelaser beam can be carried up to 30 meters on the fiber 16. Similarly,fiber 38 can carry the output of the ICP 32 about 30 meters to thespectrometer 40.

The ablation cell 20 is attached to a three-axis robot arm 72 mounted tothe cart 60, which is also controlled remotely by the operator,preferably using images relayed from a video camera mounted on theplatform or even on the probe itself. The operator controls the robotarm 72 to position the ablation cell 20 over the center of the samplingbore 68. The tubes 28 and 30 and fiber 16, a load-bearing cable 73, andother necessary electronic cables (not shown) are wound on a spool witha winch 74, which is remotely controlled to lower and raise the ablationcell 20. The sampling bore 68 contains a liner 76 (shown in more detailin FIG. 3), which can be a conventional pipe with a cut-out area, orwindow 78, through which access to the sampling site 14 is obtained.

The ablation cell 20 is lowered into the sampling bore 68 until it isadjacent to the sampling window 78. The sampling thus proceeds with theoperators at a safe distance from the sampling site 14. When sampling iscompleted, the probe 20 is withdrawn from the sampling bore 68 and theremotely controlled mobile cart 60 is returned to the trailer 62 fortransportation to the next site. If any contamination has occurred, itis generally limited to the ablation cell 20 or the immediateaccessories (i.e., cables, etc.), allowing relatively easy clean-up. Thesample 25 itself is incinerated in the ICP plasma source 34. Ifnecessary, the ablation cell 20 and accessories can be disposed of ordestroyed and replaced at relatively low cost. Further information onlaser ablation is set forth in the paper entitled "Laser Vaporization inAtomic Spectroscopy," by H. K. Dittrich and R. Wennrich, Prog. Analyt.Spectrosc., 7, 139-198 (1984), the entire contents of which are herebyincorporated by reference herein.

FIG. 5 illustrates an alternate exemplary on-site sampling system usingan ultrasonic nebulizer 80 produce an aerosol 25 from liquid material 82instead of the laser 12 to ablate a sample. In principle, whenultrasonic waves from a transducer 80 of sufficient frequency andamplitude are produced, a capillary wave action is induced in a liquidmedium 82, causing the ejection of aerosol droplets from the liquidsurface. The droplets, the dimensions of which are dependent on theultrasonic frequency and physical properties of the liquid, can beproduced with micron sized diameters. By synchronizing the transducer 80frequencies and focusing the ultrasonic waves to a single point, a wavepattern should be generated with an amplitude sufficient to provide thequantities of sample 25 required for ICP 32 analysis. A low frequency,high power, ultrasonic stephorn generator known to be suitable for thepresent embodiment is disclosed by Fassel and Dickinson, Anal. Chem, 40,1968, 247; and in U.S. Pat. No. 3,521,949. Once a representative aerosolsample 25 is generated, it mixes with the argon 24 and is transported toICP 32 for analysis. The nebulized liquid material 82 is drawn throughthe aerosol output line 30 to the ICP 32. Sample analysis proceeds asdiscussed in connection with FIG. 1, except that the laser 12 isreplaced by the nebulizer 80.

It will be understood that the above on-site sampling systems aredisclosed by way of example only and that the present in situcalibration system may be used with any on-site sampling system,including an ordinary nebulizer in which the sample passes through anaerosol nozzle.

It is to be understood that the above description is intended to beillustrative, and not restrictive. Many other embodiments will beapparent to those of skill in the art upon reviewing the abovedescription. Although the above inventions have been described inconnection with a laser ablation system, it should be apparent that theconcepts extend to an ultrasonic nebulizer or any other samplegeneration technique. The scope of the invention should, therefore, bedetermined with reference to the appended claims, along with the fullscope of equivalents to which such claims are entitled.

I claim:
 1. An apparatus for determining the concentration of an elementof interest in a sample contained in an aerosol stream, comprising:aninductively coupled plasma spectrometer for vaporizing, atomizing, andionizing at least the element of interest to emit characteristicemissions, the emissions generating an output signal in the spectrometerwith an intensity proportional to the quantity of the element ofinterest; an aerosol delivery system for delivering the aerosol streamto the inductively coupled plasma spectrometer, the aerosol streamcontaining a carrier gas and the element of interest; means formeasuring the mass flow rate of the aerosol stream; a volatilecalibration gas compound of the element of interest; and a precision gasmetering valve for introducing a series of known quantities of avolatile calibration gas directly into the aerosol stream, the meteringvalve determining the mass flow rate of the calibration gas and thespectrometer generating an output signal intensity proportional to thequantity of calibration gas and element of interest present in theaerosol so that a standard addition curve of the intensity versus thequantity of calibration gas added can be generated to determine theconcentration of the element of interest in the sample.
 2. The apparatusof claim 1 wherein the means for measuring the mass flow rate of theaerosol stream is a quartz microbalance.
 3. The apparatus of claim 1further including sample collection means for extracting at least theelement of interest from a sample site and introducing at least theelement of interest into the aerosol stream.
 4. The apparatus of claim 3wherein the sample collection means comprises a laser ablation system.5. The apparatus of claim 3 wherein the sample collection meanscomprises a nebulizer.
 6. The apparatus of claim 1 further including astepper motor for activating the precision gas metering valve.
 7. Theapparatus of claim 1 wherein the characteristic emissions comprise ionsand the spectrometer comprises a mass spectrometer.
 8. The apparatus ofclaim 1 wherein the characteristic emissions comprise electromagneticradiation and the spectrometer comprises an atomic emissionspectrometer.
 9. An apparatus for determining the concentration of anelement of interest collected at a remote sampling site, comprising:aninductively coupled plasma spectrometer for vaporizing, atomizing, andionizing at least the element of interest to emit characteristicemissions, the emissions generating an output signal intensityproportional to the quantity of the element of interest in the sample; alaser proximate the sampling site; a first optical fiber with first andsecond ends, the first end coupled to the laser and the second endpositioned proximate the remote sampling site for directing a laser beamfrom the laser onto a surface of the sampling site to abate a sample ofat least the element of interest; an aerosol delivery system containinga carrier gas for delivering at least the element of interest to theinductively coupled plasma spectrometer where at least the element ofinterest is dissociated, atomized and excited to provide itscharacteristic optical emissions; a volatile calibration gas compound ofthe element of interest; and a precision gas metering valve forintroducing a series of known quantities of a volatile calibration gasdirectly into the aerosol stream containing at least the element ofinterest, the metering valve determining the mass flow rate of thecalibration gas and the spectrometer generating an output signalintensity proportional to the quantity of calibration gas and element ofinterest present in the aerosol so that a standard addition curve of theintensity versus the quantity of calibration gas added can be generatedto determine the concentration of the element of interest in the sample.10. A method for determining the concentration of an element of interestin a sample contained in an aerosol stream, comprising the stepsof:providing an inductively coupled plasma spectrometer for vaporizing,atomizing, and ionizing at least the element of interest to emitcharacteristic emissions, the spectrometer generating an output signalintensity proportional to the quantity of the element of interest in thesample; providing an aerosol delivery system for delivering the aerosolstream to the spectrometer; measuring them ass flow rate of the aerosolstream; introducing a series of known quantities of a volatilecalibration gas of the element of interest directly into the aerosolstream using a precision gas metering valve, the spectrometer generatingan output signal intensity proportional to the quantity of calibrationgas and element of interest present in the aerosol stream; determiningthe mass flow rate of the calibration gas; and generating a standardaddition curve of the output signal intensity versus the quantity ofcalibration gas added to determine the concentration of the element ofinterest in the sample.
 11. The method of claim 10 wherein thecharacteristic emissions comprise ions and the spectrometer comprises amass spectrometer.
 12. The method of claim 10 wherein the characteristicemissions comprise electromagnetic radiation and the spectrometercomprises an atomic emission spectrometer.
 13. The method of claim 10further including the step of comparing the mass flow rate of theelement of interest with the mass flow rate of the aerosol stream todetermine the concentration of the element of interest in the sample.14. The method of claim 10 wherein the step of measuring the mass flowrate of the aerosol stream comprises diverting a portion of the aerosolstream to a means for measuring mass flow rate.
 15. The method of claim10 wherein the step of measuring the mass flow rate of the aerosolstream comprises diverting the entire aerosol stream to a means formeasuring mass flow rate prior to introducing the calibration gas. 16.The method of claim 10 further including the step of:providing samplecollection means for extracting at least the element of interest from asample site; and introducing at least the element of interest into theaerosol stream.
 17. The method of claim 16 wherein the sample collectionmeans comprises a laser ablation system.
 18. The method of claim 16wherein the sample collection means comprises a nebulizer.